The invention relates to a method and a device for the contact-free determination of the mechanical rigidity of biological tissues in body cavities (also arthroscopically) under a defined jet of liquid. It is suited for measuring the mechanical rigidity of native cartilage, articular cartilage substitute, cartilage deficiency filling, as well as of any other biological tissues. Through comparative measurements, the mechanical quality of the biological materials may be compared to the native material.
For one, the properties of articular cartilage can be determined, for example, optically (arthroscopy), histologically, and by means of imaging methods (magnetic resonance). These methods allow for distinguishing damaged cartilage from sound areas. These methods are moreover used for assessing post-operatively the quality of healing cartilage deficiencies or deficiency fillings. Since articular cartilage is exposed to high loads, its mechanical suitability is an essential criterion for the long-term function of the tissue and the entire articulation. In particular new methods for filling cartilage deficiencies such as, for example, the tissue engineering of articular cartilage, depend on the determination of the mechanical suitability of the deficiency fillings. Though in mere in-vitro tests, the basic suitability of the substitute materials is pre-operatively determinable, more decisive is the mechanical property of the deficiency filling in a post-operative situation.
To date, there exist only few approaches for the post-operative determination of the mechanical suitability of biological tissues (articular cartilage, skin). In the patent specifications U.S. Pat. No. 5,673,708, WO 95/28880, WO 98/08073, WO 97/05825, CA 2161587 and WO 93/02619, mechanical testing instruments are described by means of which the deformation of the cartilage can be determined under a manually applied load.
A disadvantage of these devices and methods, respectively, resides in that altogether relatively high loads are necessary for determining the mechanical rigidity of the articular cartilage. Due to the direct mechanical contact of the test instruments with the articular cartilage, a damage of the regenerated material in particular in reconstructed articulation faces may easily result. Previous devices did not take into account that with manual use, measurement inaccuracies may easily occur due to different test positions and contact forces. The existing methods therewith do not exclude inter-individual variations of the measurement values.
In the patent specifications WO 93/20752 and GB 2191006, the path or path variation is optically determined during a mechanical invasive inspection. This is particularly important with the determination of the relaxation behavior of the cartilage. Similar to the above-mentioned test methods, considerable local test loads arise in the latter method as well. A damage of the newly formed cartilage areas can therewith not be excluded. It may be true that extended testing for determining the relaxation behavior is moreover important in vitro, it is, however, applicable only in a limited manner in a patient measurement.
In the patent specification DE 19711516, U.S. Pat. Nos. 5,779,651, 5,246,013, 5,474,070, WO 94/05993, WO 92/21023, WO 91/07657, WO 93/17622 and U.S. Pat. No. 5,265,612, methods are described, wherein the quality of cartilage or comparable materials is determined by variations in the ultrasonic signals or the electric resistances. A prerequisite for the application of these methods is a direct coupling of the measurement and signal unit to the biological tissue. In particular in arthroscopic examinations, this cannot always be ensured. During surgical operations on a patient, coupling media, which enable a better signal transfer from the measurement device to the biological tissue may be used only in a restricted manner. For this reason, high contact forces are in each case necessary for obtaining the desired signals. With this method, the influence of differing material thicknesses (cartilage thickness) and measurement angles (angles between the incident and the reflected signal path) on the measurement result may not be excluded.
In the patent specifications DE 19650992 A, DE 36123112 A1 and U.S. Pat. No. 5,265,612, methods are described for deforming skin and other soft tissues by defined pressures. A disadvantage of these methods is that only soft samples can be tested, since the deformation is applied by air streams. In addition, in the patent specification DE 36123112 A1, the measurement is not automatically triggered with approximation to the test object. So as to be able to determine the rigidity independent of the respective user, the automatic release of the system at predefined parameters (e.g. distance, pressure), however, is essential.
The invention is based on the task of developing a method and a test device, by means of which the biological properties of biological materials (e.g. articular cartilage) can be determined without causing damage to the biological tissue and without exhibiting strong user-dependent variations in the measurement signal.
This task is solved in that biological samples in body cavities, in particular cartilage, are deformed by a jet of liquid, and that this deformation is optically detected. In a method and by means of a device according to the invention, the distance between the measurement device and the measurement surface (e.g. articulation face) is measured continuously in a predetermined measurement zone with an optical method. If this distance falls below a pre-given value, a clear jet of liquid is directed on the measurement surface at a defined pressure through the feed pipe. The deformation of the test object by the jet of liquid is detected and evaluated by the optical distance measurement system. Starting from a very low load level, the inventive device allows the iterative increase of the load up to a defined, maximally permissible deformation of the sample surface. By means of this proceeding, damages of the sample body (e.g. the articulation face) are supposed to be avoided. From the pressure of the jet of liquid, the tube diameter and the deformation of the articulation surface (distance variation), the rigidity of the cartilage is determined, and is displayed online to the user.
By means of the device realized according to the invention, the mechanical rigidity of biological tissues (e.g. articular cartilage) may also be determined in body cavities (e.g. arthroscopically) without the possibility of resulting in a direct contact of the measurement system with the sample surface, too strong a loading of the sample surface or too strong user-dependent variations of the measurement results.
Surprisingly, it has been found that very soft biological samples (e.g. even reconstructed articulation faces) under a low load ( less than 5 N), as well as very hard samples (e.g. intact cartilage) can be measured. The invention enables the contact-free and user-dependent measurement of biological tissues. The inventive device is simple to handle, of flexible application and easy to operate. Due to the secure function, there does not exist any direct or indirect risk arising from the measurement, neither for the patient nor for the user.